As background for the present invention, reference is invited to four Doherty U.S. Pat. Nos. 3,709,083; 4,022,090; 4,056,029; and 4,135,770, copies of which are being furnished herewith for convenience of examination.
In the punch press shown in Doherty--U.S. Pat. No. 3,709,083 the tooling is in a fringe flux path of the solenoid winding 82, 84 such that the tooling tends to become magnetized. There is no provision for direct cooling of the electromagnet. There is no provision shown for adjustment of stroke length.
In the punch press disclosed in Doherty--U.S. Pat. No. 4,022,090, the solenoid winding is located in the base of the machine sandwiched between a base plate and a connecting plate causing the winding to be subject to overheating and being without cooling. The base location of the bulky winding interferes with handling of materials, interferes with exit of scrap pieces, makes for inconvenient tooling set-up, and causes difficulty in adjustments of open and shut height and stroke length. In the Doherty punch press shown in the '090 patent, the tooling and guide pins and their bushings are all located in a significant fringe flux path of the solenoid winding causing them all to become magnetized. Such magnetization of tooling, guide pins and bushings causes steel shavings, filings, burr pieces and flakes and similar scraps of steel to become attracted to and to collect on these magnetized parts, leading to their rapid deterioration or actual sudden destruction. When a steel scrap of significant size becomes adhered to one side of a male or female die overlying a cutting edge, subsequent closure of the dies produces a sudden, eccentric, off-axis obstruction deflecting the male die and causing it to smash into an incorrect region of the female die with consequent destruction of the dies. Steel particles adhering to the guide pins become abrasive intrusions wedging between the moving parts and quickly wearing and soon destroying them.
In the punch press described in Doherty--U.S. Pat. No. 4,056,029, the solenoid winding is located in a movable housing which is secured to a movable tool holder. The winding together with its electrical connections moves during each stroke. Thus, the winding and its connections are subjected to a severe beating due to repeated frequent mechanical shocks of tool impact against work material. Such repeated mechanical impact stressing of the winding and its electrical connections tends to cause early electrical failure with relatively short reliable operating life. It is noted that in FIGS. 2 and 4 the moving bushings 26 bottom on stops 86 (col. 5, line 16).
Doherty U.S. Pat. No. 4,135,770 is a Division of U.S. Pat. No. 4,056,029, and is directed to a leader pin for a punch press die set having a bore in the leader pin containing an elongated axially movable button and a spring urging this movable button toward its extended position. This button is pushed inwardly during each power stroke and springs back to its extended position for pushing the movable portion of the die set back to its initial position. This complex leader pin serves as a stationary armature in cooperation with a movable solenoid winding having a movable housing as discussed above regarding the '029 patent.
In the '029 and '770 patents moving non-ball-bearing bushings 26 (FIGS. 2 and 4; often called "sleeve bushings") are shown bottoming on stops 86. During recent years ball-bearing bushings (often called "linear bearings") have been used instead of sleeve bushings in many instances in punch presses for reducing friction during the power and return strokes. The balls in such linear bearings are mounted in a cage (also called a "retainer") within a housing, and it is the housing of the linear bearing which bottoms against such stops. A problem caused by such substitution of linear bearings for sleeve bushings is the consequent downward skidding of the cage and balls, caused by their own downward momentum thereby causing rubbing wear on the guide pin during each downward stroke and return stroke with consequent rapid deterioration of the balls and cage in the linear bearing. In other words, each time the bearing housing bottoms down against the stop, the housing suddenly stops its rapid downward motion, but the cage and balls have significant downward momentum and tend to skid down. During the subsequent upstroke, the misplaced balls cannot roll freely due to the conventional retainer, and so they skid up against the guide pin with consequent undue wear of the guide pin and rapid deterioration of the components in the linear bearing.